This invention relates to reducing the effects on breakdown voltage of thermal stresses in high voltage vacuum devices. More particularly, it relates to reducing the electrostatic forces at the anode surface of such a device, which forces propel particles from the anode toward the cathode and thereby contribute to breakdown.
In high voltage vacuum devices such as x-ray tubes, the ability to withstand high voltage across the cathode-to-anode gap without breakdown is often limited by the mechanical and thermal properties of the material used for the anode. It is known from field emission theory that one of the mechanisms of breakdown involves electron beam interaction with the anode surface. Prior to breakdown, field emitted electrons are formed at the cathode. Alternatively, electrons may be generated thermionically at the cathode. These electrons traverse the cathode-to-anode gap and strike the anode. Since the electrons have gained the full potential energy associated with the voltage across the gap, upon impact with the anode they act to heat and dislodge solid or liquid particles of anode material. Movement of the dislodged anode particles has three principal effects. First, because the particles have acquired a positive charge from the anode, and because there is an electric field across the gap between the anode and the cathode, the particles move upstream in the beam of incident field emitted electrons. In their motion toward the cathode, the particles become heated, vaporized, and ionized, leaving a plasma column in their wake. This column becomes the path along which electrical conduction is established in the initial stages of breakdown. Secondly, the plasma column, and perhaps even the unvaporized dislodged particles themselves, effectively extend the anode surface toward the cathode, thereby shortening the cathode-to-anode gap. Since the electric field strength produced by voltage across the gap is inversely proportional to the length of the gap, the field strength is increased when the gap length is decreased. The third effect of movement of dislodged particles occurs when the particles completely traverse the gap and strike the cathode. The result is essentially breakdown of the dielectric gap and electrical conduction between the cathode and the anode.
In the past, the general approach to the problem of breakdown involving particles dislodged from the anode has been to use an anode material of relatively high thermal stability, such as molybdenum or tungsten. Another approach has been to coat the surface of the anode with a material having high thermal stability, such as a rhenium-tungsten alloy. While these approaches improve the dielectric performance by lengthing the time before particles become dislodged and/or reducing the number of dislodged particles, breakdown ultimately occurs by the same type of electron beam-anode interaction as described above.
Furthermore, it is known that electrical breakdown can also be caused by the presence of free particles within the evacuated volume, by the same type of mechanisms as for particles dislodged from the anode. Free particles contacting the anode surface acquire a positive charge and move toward the cathode in the same manner as particles dislodged from the anode. Movement of these charged free particles has similar effects as movement of dislodged anode particles.
Accordingly, it is an object of the present invention to reduce the effects of thermal stresses on breakdown voltage in high voltage vacuum devices.
It is a further object of the present invention to reduce the movement of dislodged anode particles in a direction toward the cathode.
It is also an object of the present invention to reduce the movement of free particles in the evacuated volume in a direction toward the cathode.